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The Cardinal Coach Line bus carrying about 45 passengers when it hit a rubber crash cushion, struck two concrete barriers and flipped on its side Thursday morning. (Published Friday, April 12, 2013)

Two people were killed and more than three dozen were injured when a private charter bus overturned on a highway outside Dallas on Thursday morning.

The bus full of seniors was headed to Choctaw Casino Resort in Durant, Okla., a passenger told NBC 5 DFW. The bus was going northbound on the President George Bush Turnpike in Irving, Texas, when it veered to right and back to the left before flipping over near Belt Line Road at about 9 a.m.

Two people were killed in the crash, said Department of Public Safety spokesman Sgt. Lonny Haschel. Their names have not been released.

Forty people were transported to area hospitals. Irving fire officials said 45 people were on the passenger list.

Sixteen patients are in critical condition, while many others sustained only minor injuries, according to emergency crews. The injured were sent to various Dallas-area hospitals, including Baylor Irving Medical Center, Las Colinas Medical Center, Parkland Memorial Hospital and Methodist Hospital in Dallas.

Bus driver Loyd Rieve is among those patients. Rieve's family told NBC 5 DFW he is in the intensive care unit in serious condition at Parkland Hospital, with injuries to his lungs, ribs and various bruises all over his body.

Injured passengers who were able to walk were loaded on a Dallas Area Rapid Transit bus for transport to Baylor Irving Hospital. Baylor staff called in extra staff to help deal with the injuries.

Dr. Paul Pepe, of Parkland Memorial Hospital, said doctors were taking a closer look at every injury.

"Being elderly, we are watching them like hawks," he said.

Dr. Alex Eastman, Parkland director of trauma, said four patients there between the ages of 66 and 80 were in critical condition.

DPS said the bus was traveling northbound when the bus drove onto the right side of the road for an unexplained reason. The bus hit a rubber attenuator and traveled back across all lanes of the turnpike and onto the grassy median before running up along the middle concrete barrier. The bus flipped over and slid on its right side, the same side as the exit doors.

Trapped passengers had to use emergency hatches on the roof and push out the windows on the left side of the bus to either climb out or await rescue by passers-by and emergency crews.

The family of the bus driver told NBC 5 DFW on Thursday that Rieve has been driving commercially, including 18-wheelers and charter buses, for more than 30 years.

They said they don't believe driver fatigue was a factor and that he is very strict about his sleep schedule. He had been in wrecks before, his family said, but added that they were "not his fault."

In the past two years, none of the company's five buses has been in a crash, the records say. But the company's two inspections over that period found violations that resulted in putting a bus and/or or a driver out of service.

The National Traffic Safety Board announced it is also sending a team of regional investigators to the scene of the bus crash.

"People Stacked on Top of Each Other"

Passenger Dan Risik told NBC 5 DFW that the charter bus had left for Choctaw Casino Resort from a Wal-Mart parking lot.

After the crash, Risik said people were laying on other people, trapping individuals underneath each other.

Risik said many of the passengers included retirees and senior citizens.

Robert Hare and Ed Cluck witnessed the aftermath of the crash and stopped to help injured passengers.

"A lot of people in shock, broken bones," Hare said. "[There are] a couple of people who are dead. They were crushed under the bus."

"You see this bus on the side of the road, and what can you do?" Hare said. "You just stop, you get out and you do whatever you can to help."

"[I] just saw this big, white smoke. My first thought, it was a street sweeper. Man, that's awfully odd, a street sweeper," Ed Cluck said. "Cars started slowing down, and you realized it was a vehicle on its side — and obviously a big one. [I] stopped, jumped out and got to the front of the vehicle. You look into [it], see who you could help, and it was just people stacked on top of each other."

Hare and Cluck helped pull passengers from the bus.

"[We] popped the roof hatches and try to get out everybody we could," Cluck said. "It was pretty bad; people screaming. There was obviously a lot of pain. It was bad."

Survivor: "I Feel Very Lucky"

"It happened pretty quick, we were going down the road and all of a sudden a crashing sound like it ran into something and the bus started to shake vehemently and then turned over on its right side," said Risik. "People were on top of people, screaming, hollering, yelling for help. It wasn't a very pleasant situation to be in."

Risik — who was seated in the middle of the bus on the right side in an aisle seat — said when the vehicle came to stop, his leg was trapped.

"My buddy I was on top of him. He was saying 'please get off me, please get off me.' Well, I couldn't, because a lady was on my leg, and she couldn't move because she was kinda in between the seat or whatever," said Risik.

The bus was carrying about 45 people, most of them senior citizens, law enforcement officials said. Risik said most of the passengers weren't wearing seat belts.

"After hearing what I've heard and seeing what I've seen in person, I feel extremely fortunate to be virtually injury-free. A few scrapes, whatever," said Risik. "Other than that I feel very lucky."